An introduction to the SignalK(SK), Marine application, is given here.
A very easy way to measure temperatures with SK is to use one wire (1W) DS18B20 (DS)temperature sensors.
In a former post the DS:s where connected direkt to the RPi GPIO:s, but in this post they are WiFi enabled via the Shelly 1(S1). Alarms are also sent when the temperature exceeds a set value.
If humidity is of interest You can use a DHT22 sensor instead of a DS sensor.